1. Field of the Invention
The present invention relates to a turbine blade airfoil for an axial-flow turbine including an intrados generating a positive pressure and an extrados generating a negative pressure, the intrados and the extrados being provided between a leading edge and a trailing edge, a turbine blade to which such turbine airfoil is applied, and a turbine blade cascade comprising an assembly of such turbine blades.
2. Description of the Prior Art
A turbine blade S and blade cascade of a conventional axial-flow turbine are shown by a dashed line in FIG. 1. The airfoil of the turbine blade S includes a leading edge LE, a trailing edge TE, an extrados Su extending from the leading edge LE to the trailing edge TE and generating mainly a negative pressure during operation of the turbine, and an intrados S1 extending from the leading edge LE to the trailing edge TE and generating mainly a positive pressure during operation of the turbine. A portion of the intrados Sl near the trailing edge TE assumes a simple concave shape having no inflection point, and the bladexe2x80x94blade distance D in the blade cascade of adjacent turbine blades S, namely, the length of a normal line drawn downwards from the intrados Sl of one of the turbine blades S to the extrados Su of the other turbine blade S is decreased monotonously in a region extending from a front throat to a rear throat.
There are conventionally known inventions relating to the shape of a trailing edge portion of a turbine blade, which have been described in Japanese Patent Application Laid-open Nos.57-113906, 7-332007 and 9-125904.
The turbine blade described in Japanese Patent Application Laid-open No.57-113906 has a construction in which a trailing edge is curved toward an extrados, or a construction in which the curvature of the extrados at the trailing edge is larger than that of an intrados. This construction ensures that the generation of a shock wave at a transonic speed is controlled to alleviate the load applied to the turbine blade and to reduce the pressure loss.
The turbine blade described in Japanese Patent Application Laid-open No.7-332007 has a corrugated unevenness at a trailing edge. This construction ensures that the distribution of flow in the radial direction of a turbine is liable to be interfered, and the proportion of speed loss due to a wake is reduced to enhance the flowing performance at each stage of the turbine.
In the turbine blade of a vapor turbine described in Japanese Patent Application Laid-open No.9-125904, a portion of an extrados at a trailing edge is cut out rectilinearly. This construction ensures that the pressure loss is reduced, while ensuring a resistance to erosion caused by the vibration applied by a vapor flow or by foreign matters within the vapor flow.
The blade S (see the broken line) of the conventional axial-flow turbine shown in FIG. 1 exhibits a sufficient performance in a state in which the flow speed along a surface of the blade is a high subsonic speed and there is no shock wave generated. However, this blade S suffers from a problem that if the flow speed at the trailing edge reaches a sonic speed, shock waves SW1 and SW2 generated from the intrados Sl and the extrados Su at the trailing edge cause a reduction in performance. Particularly, one SW1 of these shock waves interferes with a boundary layer on the extrados Su of the adjacent turbine blade S to cause a pressure loss, thereby making it difficult to enhance the performance of the entire turbine.
The present invention has been accomplished with the above circumstance in view, and it is an object of the present invention to minimize the influence of a shock wave generated from the intrados at the trailing edge of the turbine blade for the axial-flow turbine to enhance the performance of the turbine.
To achieve the above object, according to a first feature of the present invention, there is provided a turbine blade airfoil for an axial-flow turbine including an intrados generating a positive pressure, and an extrados generating a negative pressure, the intrados and the extrados being provided between a leading edge and a trailing edge, characterized in that when the position along the intrados (S1) is represented by percentage such that the position of the leading edge is represented by 0%, and the position of the trailing edge is represented by 100%, an inflection point is provided between a concave portion on an upstream side and a convex portion on a downstream side in a region extending from a position of 80% on the intrados to a rear throat.
With the above arrangement, the inflection point is provided between the concave portion on the upstream side and the convex portion on the downstream side in the region extending from the position of 80% on the intrados to the rear throat. Therefore, it is possible to disperse a shock wave generated from the intrados at the trailing edge to prevent the generation of a strong shock wave, thereby reducing the pressure loss caused by the shock wave.
According to a second feature of the present invention, there is provided a turbine blade for an axial-flow turbine, which turbine blade is obtained by applying the turbine blade airfoil according to the first feature to at least a portion of the turbine blade in a span direction.
With this arrangement, it is possible to enhance the degree of freedom of the design of the turbine blade by using the turbine blade airfoil according to the present invention and an existing turbine blade airfoil in combination as desired.
According to a third feature of the present invention, there is provided a turbine blade cascade comprising an assembly of turbine blades having the turbine blade airfoil according to claim 1, characterized in that the length of a normal line drawn downwards from an intrados of one of a pair of adjacent turbine blades to an extrados of the other turbine blade has at least one maximum value in a region extending from a front throat to a rear throat of the one turbine blade.
With the above arrangement, the length of the normal line drawn downwards from the intrados of one of the pair of adjacent turbine blades to the extrados of the other turbine blade has at least one maximum value in the region extending from the front throat of the one turbine blade to the rear throat. Therefore, a speed-reducing area can be formed on the extrados generating the negative pressure to promote the transition from a laminar flow boundary layer to a turbulent flow boundary layer, thereby preventing the seperation of the boundary layer caused by the interference with a shock wave to reduce the pressure loss.
According to a fourth feature of the present invention, in addition to the third feature, there is provided a turbine blade cascade for an axial-flow turbine characterized in that the maximum value is equal to or smaller than 110% of the length of the normal line at the front throat.
With the above arrangement, the maximum value of the length of the normal line drawn downwards from the intrados of the one turbine blade to the extrados of the other turbine blade is equal to or smaller than 110% of the length of the normal line at the front throat. Therefore, a smooth transition from a laminar flow boundary layer to a turbulent flow boundary layer can be achieved.